1. Field of the Invention
The present invention relates to Class D switching audio amplifiers. More particularly, the present invention relates to a Class D switching audio amplifier making use of four state modulation, input-to-output drive and feedback signal isolation, a dual topology output filter, and a low inductance board layout.
2. Description of the Prior Art
It is often desirable to amplify audio signals using a Class D switching audio amplifier. Basic circuit layout of the Class D amplifier is substantially similar to that of linear amplifiers, such as Class A, B, and AB, with a major difference being in the signals provided to an output stage. Rather than feeding an audio waveform directly to the output stage, as is done in linear amplifiers, the Class D amplifier first feeds the audio waveform into a Pulse Width Modulator (PWM) circuit which feeds modulated pulses to the output stage. By quickly switching the output stage completely on and completely off with varying pulse widths, the Class D amplifier is able to recreate waveforms of almost any shape, and, by filtering the switching output, sound is produced by a loudspeaker connected thereto. In practice, the pulses are fed to the output stages at a frequency between 100 and 300 kHz, or 100 to 300 thousand pulses per second, which is required to produce a smooth waveform at the loudspeaker.
An advantage of the Class D amplifier is that the output stage transistors are switched either completely on or completely off. Amplifier topologies that operate in a partially on state, such as Class A and AB, act like resistors and produce heat, thereby wasting energy. Thus, Class D amplifiers are substantially more efficient than non-switching linear amplifiers. Higher efficiency and less waste heat allows the Class D amplifier to utilize a smaller power supply and to be offered in a more compact package than a comparable linear amplifier.
Unfortunately, existing Class D amplifier designs suffer several disadvantages, including disadvantages related to modulation, isolation, feedback, and board layout. Existing Class D amplifier designs incorporate a full H-bridge output stage and use a single PWM signal to derive four FET gate drive signals providing two H-bridge switch states. Both H-bridge switch states result in a differential voltage across the outputs leading to current flow through the load. These two-state Class D amplifiers typically compare a reference triangle waveform to an audio error waveform (audio feedback) using a single comparator. The output of the comparator is a single PWM signal with the same frequency as the reference triangle waveform. The PWM signal is then passed through a logic circuit that generates four drive signals used to drive the H-bridge, resulting in a 180xc2x0 phase difference between outputNEG and outputPOS. Thus, a differential voltage is always present at the output causing power to be lost via the loudspeaker or low pass filter even in the absence of an audio input to the amplifier.
Existing Class D amplifiers typically require large power transformers to accommodate a relatively inefficient output stage and to meet government regulations requiring high voltage isolation between AC mains and all user-accessible inputs and outputs. This isolation is typically achieved by incorporating one or more power transformers between the AC mains and the input and output stages. Unfortunately, such power transformers are large and expensive. Furthermore, because 99% of any incoming power is required to drive the output stage and the loudspeakers connected thereto, a power transformer isolating the output stage must be substantially larger than a power transformer isolating the input stage.
Even in applications where the outputs are not user-accessible, no effort is typically made to isolate the input stage from the output stage. Where input-to-output isolation is attempted, small-signal audio transformers are typically used. Unfortunately, these transformers suffer from limited frequency response, making implementation difficult.
Typical output efficiencies for prior art linear amplifiers are approximately 60%, with the remaining 40% of supplied power being dissipated as heat. Consequently, expensive heat-sinking is required, and large, expensive power transformers are needed to deliver 66% more power than the desired output power of the amplifier. With the development of Class D amplifiers, output efficiencies increased to 85%, thereby reducing power supply requirements and waste heat. Unfortunately, expected theoretical efficiencies of 90+% for the Class D amplifier have not been achieved, due primarily to the many problems and disadvantages set forth herein.
Existing high-power Class-D amplifier designs incorporate a control or feedback loop to minimize distortion. Conventional control theory requires filtering, attenuating, and summing the output signal with the input signal. This typically involves a feedback loop comprising a differential RC low pass filter, followed by an attenuating differential amplifier, and then a summing amplifier to combine the feedback signal with the input signal. For high power applications where common-mode voltages can exceed 70 Vdc, precision matching of feedback resistors is a critical concern. Resistor tolerances greater than 1% in the differential amplifier and the RC low pass filter sections result in reduced common-mode rejection, potentially damaging voltages at the differential amplifier, and degraded product reliability. The RC low pass filter is required to attenuate the PWM switching energy and to pass the audio signal to the differential amplifier. This can result in decreased efficiency as power is lost in the RC low pass filter even in the absence of an audio input signal. High power applications require the use of high power resistors ( greater than 1W) that can effectively dissipate the switching energy. Unfortunately, precision matching and increased power handling requirements for the RC low pass filter resistors result in increased cost and size. For example, surface mount 1W 1% resistors are 7.5 times larger and 18 times more expensive than standard xc2xcW 5% surface mount resistors.
Existing Class-D amplifier designs incorporate pairs of multi-pole differential LC low pass filters to filter the ever-present differential switching output voltage. Typical multi-pole differential LC filter designs dissipate a majority of attenuated energy in the first LC low pass filter pair. No advantage is gained from common-mode filtering because the output of the H-bridge continues to be a differential voltage. As a result, high power designs are required to incorporate expensive high power inductors that can dissipate the switching energy even when no audio input signal is present.
Existing Class D amplifiers typically exhibit high harmonic distortion above 1 kHz as a result of pulse transient damping issues and poor triangle waveform damping generation. Excessive pulse undershoot and overshoot result from high inductance board layouts and power supplies. Some existing designs attempt to reduce pulse overshoot and undershoot on H-bridge outputs by incorporating large, expensive RC snubbers. Such undershoot and overshoot can degrade reliability for many standard FET driver ICs such as Harris"" HIP4080A. Additionally, pulse transient damping issues also lead to increased EMI emissions that increase the cost of shielding the amplifier.
Triangle waveform generation has always been a source of distortion in Class D amplifier designs. Triangle waves are typically generated using RC oscillators made of operational amplifiers or logic gates. These Class D amplifier designs suffer from high frequency noise superimposed on the triangle waveform; in turn, the high frequency noise results in increased harmonic distortion. Thus, existing Class D amplifiers typically exhibit undesirable harmonic distortion much greater than 0.5%.
Due to the above-identified and other problems and disadvantages in the art, a need exists for an improved Class-D audio switching amplifier.
The present invention overcomes the above-identified as well as other problems and disadvantages in the art of Class D and linear audio amplifiers by providing a Class D switching amplifier operable to provide increased efficiency, increased reliability, and reduced distortion through use of four state modulation, input-to-output driver and feedback signal isolation, dual topology output filtration, and a low inductance board layout. Though not limited thereto, the amplifier is particularly ideal for applications without user-accessible outputs, such as powered loudspeakers, wherein isolation of input-to-output drive and feedback signals allows for the elimination of large expensive power transformers required by the prior art. Furthermore, though not limited thereto, the amplifier is particularly ideal for high-power applications involving, for example, 50W or more.
The preferred Class D switching amplifier broadly comprises an input stage; a triangle stage; a gate drive stage; an output stage; a filter stage; and a feedback stage. The input stage is operable to receive first and second feedback signals, FDBK_P and FDBK_N, and an audio input signal, AUDIO_IN, and to therefrom derive first and second error signals, ERROR and ERROR_INV. The error signals represent the combined audio input and error for both positive and negative swings.
The triangle stage is operable to derive a low noise triangle waveform, TRIANGLE, having reduced high frequency noise that might otherwise lead to excessive distortion.
The gate drive stage is operable to generate four optically isolated gate drive signals, DRV_Q1, DRV_Q2, DRV_Q3, and DRV_Q4. Within the gate drive stage, the ERROR signal and the TRIANGLE waveform are also compared to produce an output signal, PWM_A; and the ERROR_INV signal and the TRIANGLE waveform are compared to produce an output signal, PWM_B. The PWM_A and PWM_B signals are then input, respectively, to first and second optoisolators. The optoisolators preferably allow isolated pulse transmission with minimal delay and pulse width distortion. The outputs of the optoisolators are taken directly to produce, respectively, the DRV_Q2 and DRV_Q3 gate drive signals; and inverted to produce, respectively, the DRV_Q1 and DRV_Q4 gate signals.
The output stage is operable to receive the gate drive signals and to derive therefrom intermediate output signals, OUT_HP and OUT_HN, and broadly comprises first and second H-bridge halves which combine to form a full H-bridge.
The filter stage is operable to reduce EMI emissions by attenuating switching energy, and is essential for series connecting, or xe2x80x9cdaisy chainingxe2x80x9d, the floating output stages of multiple instances of the Class D switching audio amplifier. The filter stage receives as input the OUT_HP and OUT_HN signals, and broadly comprises a four-pole LC low pass filter combining common-mode filter topology for lowering inductor current in the absence of an audio input signal, and differential filter topology for attenuating high frequency differential signals. The filter stage provides final output signals, OUT_P and OUT_N, to drive the loudspeaker or other load.
The feedback stage is operable to provide the processed feedback signals, FDBK_P and FDBK_N, to the input stage, and broadly comprises first and second optoisolators 84,86 and first and second RC low pass filters 88,90. Within the feedback stage, the OUT_HP and OUT_HN signals produced by the output stage are optoisolated and filtered through RC low pass filters to result in the FDBK_P and FDBK_N signals.
As mentioned, the present invention introduces a unique four state modulation scheme that advantageously increases efficiency and allows for common-mode filtering to reduce loss during no-audio conditions. Using the four state modulation scheme of the present invention, in the absence of an audio input signal the H-bridge outputs are common-mode (in phase) and no current is delivered to the load. In the presence of an audio input signal, the H-bridge outputs differentially drive current through the load at double the frequency of the triangle waveform.
The input stage is isolated from the output stage using optoisolators. Alternatively, small signal transformers may be used in place of the optoisolators; however, the optoisolators, being more cost and space effective, are preferred. Many available optoisolators provide fast data transmission while minimizing pulse distortion effects. By isolating the input from the output, applications without user-accessible outputs can advantageously eliminate expensive high power transformers commonly found in existing amplifiers, thereby resulting in an estimated 75% weight savings and 40% cost savings over typical prior art amplifiers.
Furthermore, isolating the input stage from the output stage advantageously allows the output stage to float with respect to the chassis or input ground, which, in turn, allows for series connecting or xe2x80x9cdaisy chainingxe2x80x9d multiple amplifiers to increase power delivered to the loudspeaker. Another benefit of floating the output stage is reduction of typical Class D chassis referenced DC voltage present at the amplifier output.
Additionally, the present invention improves upon prior art Class D feedback topology by isolating feedback signals and referencing the RC low pass filter to the input stage ground. This improvement eliminates potentially damaging differential and common-mode voltages present in the feedback circuit. As a result, precision resistor matching is no longer required, and less power is lost in the RC low pass filter. Thus, isolating the feedback signals substantially reduces costs and increases efficiency and design reliability.
The filter stage includes an LC low pass output filter operable to attenuate the high frequency switching, pass the amplified audio signal, reduce radiated emissions, and smooth the output current. In prior art Class D amplifiers, differential LC low pass filter designs are used with 3 dB cutoffs at no less than 25 kHz. Regardless of whether audio is present at the output or not, the filter is absorbing energy at the switching frequency. With prior art modulation schemes no advantage was gained from common-mode filtering because the output of the H-bridge was a differential voltage waveform. As a result, high power designs were required to incorporate expensive high current, low resistance inductors in the LC low pass filters that could absorb the switching energy with or without an audio signal present.
The modulation scheme of the present invention results in a common-mode voltage in the absence of audio that allows for use of a combination common-mode and differential LC low pass filter constructed with inexpensive 5022 series surface-mounted inductors. The first two-pole LC low pass filter combination is arranged in a common-mode topology; the second two-pole LC low pass filter combination is arranged in a differential topology. Through use of a four pole combined common mode and differential LC output filter, inductor current is reduced in the absence of an audio signal. With the first two-pole combination typically absorbing more power, a common-mode topology results in less power dissipation by reducing inductor current 37% over prior art filters. By incorporating a differential second two-pole combination, the filter maintains beneficial rejection of high frequency differential signal components.
The present invention utilizes a unique low inductance board layout and modularization that advantageously lowers pulse overshoot and undershoot, leading to reduced distortion, reduced radiated emissions, and increased efficiency. The low inductance design allows for elimination of expensive RC snubbers common in prior art Class D amplifiers. With the improved board layout, harmonic distortion has been reduced to less than 0.2% typical at 200Wrms. Furthermore, the unique board layout reduces overall size and allows for small lightweight construction.
These and other important features of the present invention are more fully described in the section titled DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT, below.